1. Field of the Invention
The present invention relates to a method and apparatus for improving the uniformity of a coating material after it has been applied to a charge-retaining material, in general, and to such apparatus for improving the uniformity of a coating material that has been applied to a moving web of such material, in particular.
2. Description of the Prior Art
In the manufacture of various coated products, it is often essential that coating materials applied to such products be of uniform thickness and/or have a smooth or planar surface. In, for example, the continuous manufacture of coated photographic sheet material, a nonuniform thickness coating applied to a moving web of such material would require considerably more drying time for drying the thicker portions of said nonuniform coating than would be required for drying the thinner portions of said nonuniform coating. In addition, a temperature gradient that is optimum for drying said thicker coating portions is often excessive for optimum drying of said thinner coating portions. Drying time is usually the major factor limiting maximum production rates of many coated products. Also, many properties of photographic film such as sensitivity to light, color saturation, etc., for example, can be adversely affected when constructed with nonuniformly coated sheet materials.
Mechanical devices generally employed in the web coating art, such as doctor blades, scrapers and the like, have controlled the uniformity of web coating thickness to a limited degree. However, in the production of photographic film, for example, such contact devices have a propensity for inducing surface defects in the film coatings and in addition, these contact devices very often have a detrimental effect on the sensitometry of a finished photographic film product.
One of the most effective coating thickness control techniques in present day use in the coating industry involves the employment of an electrostatic field to assist in the uniform deposition of coating materials on products to be coated. In the production of photographic film, for example, a web or sheet of material to be coated is passed between an electrically conductive support or backing roller and a coating applicator from which coating materials can flow onto a particular surface of said web. An electrostatic field is established across the gap between the coating applicator and said backing roller by a high voltage power supply whose output terminals are connected between said applicator and said roller. The electrostatic field in said coating causes a coating of uniform thickness to be deposited on a particular web surface. The magnitude of the voltage established between said applicator and said roller is normally less than that required to generate corona, but often exceeds 3KV DC.
In patent application Ser. No. 222,331, filed in the name of S. Kisler, et al., and on the same date as the present application, now U.S. Pat. No. 4,457,256, an electrostatic coating-gap assist method and apparatus are described wherein an electrostatic charge is placed on material to be coated prior to and/or when said material is remote from the gap wherein the actual coating operation takes place. A relatively intense electrostatic field is produced between the electrostatically charged material to be coated and an electrically conductive reference member connected to a low or ground potential as said electrostatically charged material is moved through the coating gap between a support or backing roller and in close proximity to said reference member for coating purposes. The reference member may be formed by the applicator, the coating fluid or by a completely separate member. The electrostatic field causes a coating layer of uniform thickness to be deposited on the material to be coated across a wide range of coating gaps without presenting an explosion or shock hazard to personnel, and without causing damage to or being subject to interruptions by imperfections in the material to be coated.
Electrostatic fields utilized in a manner such as those described above can greatly improve the thickness and/or surface uniformity of a layer of coating material. However, the use of an electrostatic field for coating improvement purposes will often cause changes in coating properties such as surface tension and/or the residual electrostatic charge on the material to be coated, that can limit the extent to which coating uniformity can be improved with an electrostatic field. Electrostatic charges present on a coated material, or coating fluid on a coating material having an electrostatic field related change in such properties as surface tension, etc., for whatever reason or reasons, can also limit the extent to which the uniformity of a coating material can be improved.